1. Field of the Invention
The present invention relates to a demodulator and to a communications system for use in wireless communications.
2. Description of Related Art
A method of improving the performance of a conventional receiver by incorporating coherent detection in the Viterbi decoder of the receiver is taught, for example, in xe2x80x9cViterbi demodulation with a phase tracking functionxe2x80x9d (Serizawa, Asakawa, and Murakami, The Institute of Electronics, Information and Communication Engineers B-II, Vol. J77-B-II No. 12, pp. 767-779, December 1994). This related art is described further below with reference to the accompanying figures.
A Viterbi decoder without a coherent detection function is described first as exemplary of the related art. FIG. 18 is a block diagram of a receiver 200 comprising a conventional Viterbi decoder 220. A coherent detector 210 first detects phase synchronization of a signal received by the receiver 200, and then generates a recovered carrier phase synchronized to the received signal. The signal sign is determined based on the phase difference between the recovered carrier and the reception signal.
How the coherent detector 210 detects the signal sign is described next with reference to FIGS. 19A and 19B using by way of example a binary phase shift keying (BPSK) modulation method. In BPSK modulation, the code of a received signal is 0 if the absolute value of the phase difference between the recovered carrier and the received signal is less than or equal to xcfx80/2; if greater than or equal to xcfx80/2, the symbol is 1. That is, if the recovered carrier phase and the carrier phase of the received signal are the same, signals in the shaded area in FIG. 19A are 0, and all other signals are 1.
If we assume that the transmitter sends a 0, a signal will be received at reception signal point xe2x80x9caxe2x80x9d under ideal, noise-free conditions. However, received signals are typically not received under such ideal conditions and are detected with some amount of noise. As a result, a signal that should be received with signal phase at point xe2x80x9caxe2x80x9d may be received at point A due to this noise as shown in FIG. 19A. If phase synchronization of the recovered carrier and the received signal is perfect, point A will remain in the shaded area even in this case and the signal will be correctly detected as a 0, thus not deteriorating the bit error rate characteristics.
In general, however, phase synchronization between the recovered carrier and the carrier wave of the received signal is not perfect due, for example, to the effects of phase shifting caused by fading in the typical channel of the mobile communications. For example, when the phase of the recovered carrier is different from the carrier wave of the received signal as shown in FIG. 19B due to fading, the above-noted point A will no longer be within the shaded area. The signal at point A will therefore be erroneously detected as a 1, and the bit error rate characteristics of the receiver is deteriorated. That is, if the carrier recovery circuit of the coherent detector does not generate a recovered carrier that is phase synchronized to the received signal, the bit error rate characteristics is likely to be deteriorated.
The operation of the above-noted coherent detector 210 is described next with reference to FIG. 20, a block diagram of the coherent detector 210. As shown in FIG. 20, this coherent detector 210 comprises a multiplier 211 and carrier recovery circuit 212. A received signal input to the coherent detector 210 is applied to the multiplier 211 and to the carrier recovery circuit 212. The carrier recovery circuit 212 generates the recovered carrier as further described below. The multiplier 211 applies coherent detection using the received signal and the recovered carrier output from the carrier recovery circuit 212.
Operation of the carrier recovery circuit 212 is described next below with reference to FIG. 21, a block diagram thereof. As shown in FIG. 21 this carrier recovery circuit 212 comprises a doubler 212a, multiplier 212b, loop filter 212c, voltage-controlled oscillator 212d, PLL circuit 212e, and divide-by-two frequency divider 212f. The received signal input to the carrier recovery circuit 212 is doubled by the doubler 212a. By thus doubling the received signal, the doubler 212a obtains a constant phase regardless of the signal sign. That is, if the signal is 0, the signal phase is 0, and if the signal is 1, the signal phase is xcfx80 with BPSK modulation. By doubling, therefore, signal phase is 0 and 2xcfx80, respectively, and matches.
Output from the doubler 212a is input to a PLL circuit 212e comprising multiplier 212b, loop filter 212c, and voltage-controlled oscillator 212d. Output from the PLL circuit 212e is a high SNR signal phase synchronized with the transmission signal carrier wave. The recovered carrier can thus be obtained by divide-by-two frequency division by the frequency divider 212f. 
A problem with the above-noted carrier recovery circuit 212 is the phase ambiguity with doubling. This is further described below with reference to the signal space diagrams in FIG. 22A to 22D FIG. 22A shows a received signal distribution. In FIG. 22A the received signal will be distributed within the range of A if the sign of the transmitted signal is 0, and B if the sign is 1, as a result of noise. If the received signal is doubled, the output of doubler 212a can be represented by C in FIG. 22B.
If the doubled signal is then input to PLL circuit 212e to increase the S/N ratio, the distribution range of PLL circuit 212e output will be narrowed as indicated by Cxe2x80x2 in FIG. 22C due to noise. If the output of PLL circuit 212e is then frequency divided by two by the frequency divider 212f, the recovered carrier may be one of two states Axe2x80x2 and Bxe2x80x2 in FIG. 22D. That is, the recovered carrier has two stable points with indefinite phase offset 180xc2x0. It may therefore not be possible to correctly reproduce the carrier wave phase due to doubling by the carrier recovery circuit 212.
In the case of BPSK modulation, the incorrect carrier wave phase will be shifted xcfx80 from the correct carrier wave phase. Shifting the carrier wave phase in this way deteriorates the bit error rate characteristics. Various methods of solving this phase detection problem can be used to avoid this drop in performance, including differential coding, and carrier wave reproduction using a known pattern. Differential coding, however, invites a drop in the SNR, and methods using a fixed pattern invite a drop in transmission efficiency.
The coherent detector 210 in FIG. 18 outputs to the Viterbi decoder 220, the operation of which is described next below with reference to the block diagram thereof shown in FIG. 23. As shown in FIG. 23, the Viterbi decoder 220 comprises a plurality of Viterbi decoding units 221, comparison and selection circuit 222, path metric memory 223, and path memory 224. It is to be noted that transition from state k to state m only in the Viterbi decoder 220 is considered below as operation is the same for other state transitions.
When a received signal is supplied to the Viterbi decoding units 221 at a particular symbol time, a branch metric is obtained by comparing the received signal with an ideal received signal, known as a replica signal, corresponding to a state transition. This replica signal is described next. Assuming a transition from state k to state m, the replica signal will be the signal output from the encoder when the transmission-side convolutional encoder changes from state k to state m. This value is uniformly determined by the convolutional encoder for each state, and is thus known to the receiver.
It is to be noted that the signal output when the transmitter""s convolutional encoder changes from state k to state m is identical to this replica signal under ideal conditions free of noise and fading. In other words, the replica signal is the ideal received signal if it is assumed that the received signal was sent when the convolutional encoder changed from state k to state m. It is therefore possible to determine the probability of a branch path from state k to state m based on how closely a received signal matches a replica signal. Because the branch metric is a value determined by comparing the replica and received signal, it can be used as an indicator of the probability of a transition in the received signal from state k to state m.
The branch metric is then added to the path metric stored in the state k path metric memory 221a. The state k path metric is a value indicative of the probability of state k, including past transitions. In other words, by adding the probability, that is, the branch metric, of a transition from state k to m to the state k path metric, it is possible to determine the probability of a path including past transitions from state k to m.
The added metric is then input to he comparison and selection circuit 222 for a comparison and selection operation using the metrics for other state transitions to determine the surviving paths to state m at the next symbol time. More specifically, the probability of a transition path from state k to state m is compared with the path probability of other state transitions to select the most-likely path as the state m path at the next symbol time.
When the most-probable path for a transition from state k to state m, for example, is selected, the metric for this transition from state k to m is stored to the state m path metric memory to update the path information. It is to be noted that the Viterbi decoder 220 has a path memory 224 for each state, that is, memory for storing previous state transitions for each state. When the comparison and selection circuit 222 selects a most-probable state, the selected state is then stored to path memory 224. The stored state transitions are equivalent to the most-probable decoder inputs for each state.
By thus selecting the most-probable path for all states to update the path-metrics and store the state transitions to path memory 224, the most-probable state and the most-probable path back through the trellis from the most-probable state can be selected after all received signal have been received. The path memory content in the most-probable state is the most-probable decoding result, that is, the result with the highest probability. As described above, the Viterbi decoder 220 estimates the most likely path with respect to the convolutional evcoder.
A Viterbi decoder having a coherent detection function is described next below. FIG. 24 is a block diagram of a receiver 201 having a Viterbi decoder 230 with a coherent detection function according to the above-noted paper by Serizawa, et al. This receiver 201 inputs a received signal directly to the Viterbi decoder 230 without performing any detection, and obtains a demodulated result from the Viterbi decoder 230. This Viterbi decoder 230 applies different phase correction to each state of a conventional Viterbi decoder.
Before decoding, phase correction is applied to each state of the received signal input to the Viterbi decoder 230. This phase correction is equivalent to coherent detection. Viterbi decoding is then applied to the phase corrected received signal, the path metric is updated along the most-probable path, and the phase correction level is adjusted. By thus matching phase correction to the Viterbi decoding state, and updating phase correction in conjunction with the path, coherent detection along the most-probable path can be performed simultaneously to decoding. It is thus possible to avoid the phase detection problem that occurs with doubling as noted above, and reliable coherent detection is possible even under extremely low SNR conditions.
Operation of this conventional Viterbi decoder 230 is further described below with reference to FIG. 25. FIG. 25 is a block diagram used to describe transition from state k to state m in a Viterbi decoder 230 having a coherent detection function according to the related art. As shown in FIG. 25, this Viterbi decoder 230 comprises a plurality of Viterbi decoding units 231, a comparison and selection circuit 232, path metric memory 233, and path memory 234. A state k input signal is first phase corrected using a phase correction level specific to that state, that is, phase correction is different for each state. More specifically, if the phase correction factor is Øk, the input signal is multiplied by exp(xe2x88x92jØk).
The Viterbi decoder 230 thus applies phase correction to the received signal using different phase correction for each state. The coherent detector 210 described above compares and detects the phase of the recovered carrier and the received signal. Phase correction in this Viterbi decoder 230, however, achieves coherent detection equivalent to coherent detector 210 using a phase Øk recovered carrier.
Using this phase-corrected signal and a replica signal, the branch metric corresponding to a transition from state k to state m is then obtained using the same method as a conventional Viterbi decoder 220. The comparision and selection circuit 232 then compares the metrics for transitions from other non-k states to state in using the metrics of the path metrics added to the branch metrics to select the most-probable path. The selected most-probable path is then used to update the path metric.
In conjunction with this metric comparison and selection operation, the phase-corrected received signal is supplied to the phase error calculating circuit 231a. The phase error calculating circuit 231a compares the phase of the-supplied received signal with the replica signal to obtain the phase error. Coherent detection phase error can be obtained by this phase comparison with the replica signal without extracting the modulation component because the replica signal is an ideal received signal.
In other words, the Viterbi decoder 230 having a coherent detection function in this example can detect phase error without frequency doubling. It is therefore possible to avoid performance degradation resulting from such doubling. A phase correction candidate for state m can also be obtained by multiplying this phase error by gain xcex1, and then adding the result to phase correction factor Øk for state k.
The state m phase correction candidate is supplied with the metric to comparision and selection circuit 232, and phase correction with the larger metric is chosen for phase correction in state m. This operation assures that phase, correction is updated according to the largest metric, that is, the most-probable path. Coherent detection following the most-probable path for Viterbi decoding can thus be achieved by applying different phase correction for each state and changing phase correction as decoding progresses. It is therefore possible to avoid the effects of frequency multiplying in a conventional Viterbi decoder not having a coherent detection function, and an improvement in the bit error rate characteristics can be expected.
As does the above-noted Viterbi decoder 220 not having a coherent detection function, this exemplary Viterbi decoder 230 selects the most-probable state and the most-probable path tracing back from this most-probable state after all symbols in the received signal have been received. The content of the path memory for this most-probable state is output as the decoded result.
An advantage of this Viterbi decoder 230 having a coherent detection function is that better performance can be achieved in comparison with a device in which the Viterbi decoder and coherent detector are separate.
It is to be noted, however, that when this exemplary Viterbi decoder 230 is used in a time diversity communications system, the Viterbi decoder 230 has no function for diversity combining after phase correction. It is therefore necessary to apply time diversity combining before phase correction, that is, before signal-input to the decoder. This means that coherent detection occurs after diversity combining.
A problem with time diversity systems in mobile communications in which phase shifts are introduced by, for example, fading is that because the combined signals are received at different times, it is not possible to generate a combined signal in which the received signal phase is correctly reproduced if diversity combining occurs before detection. This problem is described more specifically below.
It is to be noted that this problem is considered below with reference to a time diversity system in which the transmitter sends the same information-bearing signal offset time T, and the receiver then combines these signals according to the transmission timing T. FIG. 26 is a block diagram of an exemplary system.
Referring to FIG. 26, the transmitter 240 applies the transmission signal directly to a parallel-serial converter 241, and applies the same transmission signal to a delay 242 for delaying the signal an N-bit time T before then applying the signal to the parallel-serial converter 241. The parallel-serial converter 241 then multiplexes the two input signals and outputs data at a rate twice that of the input signals. It will thus be obvious that a time diversity system sends the same data twice with a specific time delay between the two transmissions.
The parallel-serial converter 241 outputs to a BPSK modulator 243 for BPSK modulation. The signal is further amplified using, for example, a radio frequency amplifier (not shown in the figure), and then transmitted from an antenna.
The receiver 250 first amplifies a radio wave received through the antenna using, for example, a radio frequency amplifier (not shown in the figure), and passes the amplified signal to a coherent detector 251. The received signal is then detected by the coherent detector 251, supplied to a serial-parallel converter 252, and demultiplexed to the two data sequences corresponding to those multiplexed by the transmitter.
The signal sequence that was not delayed time T by the transmitter is supplied from the serial-parallel converter 252 to a delay 253. The delay 253 thus delays the signal the same N-bit time T, and the delayed signal is supplied to a combining circuit 254. The transmission signal that was delayed time T by the transmitter is input directly from the serial-parallel converter 252 to the combining circuit 254. The combining circuit 254 combines the two input signals, and the combined signal is then passed to detector 255 to determine whether the signal is a 0 or 1.
FIG. 27 is a signal space diagram of the received signal before it is input to the coherent detector 251 of the above-described time diversity system. Signal A in FIG. 27 is the received signal at time t, and vector a is the carrier phase vector at time t. Signal B is the signal received at time T after signal A was received, that is, the signal received at time t+T, and vector b is the carrier phase vector at time t+T. A small phase difference between a and b means that phase shift as a result of fading during time T is small, and thus indicates that carrier phase shift is small.
Signals A and B are the same-information signals transmitted at a spacing of time T as noted above. Both signals are here assumed to be a 0, that is, signals with the same phase as the carrier wave. If A and B are then combined with equal gain, the result will be C. Because the carrier phase shift is small, the coherent detector 251 can use either a or b for the detection of C, and in both cases will detect a code of 0, that is, the correct result.
However, if a signal Bxe2x80x2 is received with a carrier phase vector bxe2x80x2 as a result of phase shift induced by fading during time T, combining signals A and Bxe2x80x2 will result in a signal Cxe2x80x2. As shown in FIG. 27 the amplitude of this signal Cxe2x80x2 is lower than that of C. If the signal is detected using vector a, the phase difference between Cxe2x80x2 and a will be xcfx80/2 or greater, and the signal will thus be falsely detected as a 1.
This means that if diversity combining is performed before detection in a time diversity system, phase shifting caused by fading can prevent diversity combining from being correctly performed, leading to decreased diversity gain and significant degradation in bit error rate performance. On the other hand, diversity combining must be performed before phase correction in a Viterbi decoder having a coherent detection function.
A problem with applying a conventional Viterbi decoder having a coherent detection function in a time diversity system, therefore, is that diversity combining must be performed at a stage before input to the decoder, and there is a deterioration in performance under certain fading induced phase shift conditions.
Therefore, with consideration for the above mentioned problem, it is an object of the present invention to provide a demodulator and a communication system in which there is no deterioration in performance under certain fading-induced phase shift conditions.
To achieve this object, a demodulator for demodulating a multiplexed data sequence containing a plurality of data sequences of a same content multiplexed with a time difference inserted therebetween, comprises: a phase correcting means for correcting a phase of the multiplexed data sequence; a diversity combining means for separating the multiplexed data sequence output from the phase correcting means into a plurality of data sequences, removing said time difference, and combining said data sequences; and a Viterbi decoding means for Viterbi decoding the diversity combined signal output from the diversity combining means.
The phase correcting means preferably comprises: phase correction memory for storing phase correction factor data; and a multiplier for multiplying the multiplexed data sequence with phase correction factor data read from the phase correction memory.
Yet further preferably, the diversity combining means comprises: a demultiplexing means for separating the multiplexed data sequence into a plurality of data sequences, and outputting the plurality of data sequences; a delay means for delaying at least one of the plurality of data sequences output from the demultiplexing means a delay time equal to the time difference; and a diversity combiner for combining a data sequence delayed by the delay means, and a data sequence input from the demultiplexing means without being delayed by the delay means.
Yet further preferably, the diversity combiner comprises: an absolute value detector for detecting the absolute value of an input data sequence; and a vector adder for weighting the data sequence based on the absolute value detected by the absolute value detector, and then combining the data sequence.
Alternatively, the diversity combiner preferably comprises: a level detector for detecting the received signal level of an input data sequence; and a data sequence selector for selecting a data sequence delayed by the delay means, or a data sequence output from the demultiplexing means and not delayed by the delay means, based on the received signal level detected by the level detector.
The present invention further provides a communication system comprising a transmitter for modulating and transmitting a supplied signal, and a receiver for receiving a signal transmitted by the transmitter and demodulating the received signal. The transmitter of this communication system comprises: a convolutional encoding means for convolutional encoding a supplied signal and outputting convolutional coded data sequences; a multiplexing means for branching a data sequence output from the convolutional encoding means into a plurality of data sequences, and multiplexing the data sequences with a time difference inserted therebetween; and a modulation means for modulating a multiplexed data sequence generated by the multiplexing means to generate a transmission signal. The receiver in this communication system comprises: a phase correcting means for correcting a phase of a received signal; a diversity combining means for separating a signal output from the phase correcting means into a plurality of data sequences, removing the inserted time difference, and combining the data sequences; and a Viterbi decoding means for Viterbi decoding the combined signal output from the diversity combining means.
In a further communication system comprising a transmitter for modulating and transmitting a supplied signal, and a receiver for receiving a signal transmitted by the transmitter and demodulating the received signal according to the present invention, the transmitter comprises: a second convolutional encoding means for convolutional encoding a supplied signal at a xc2xc coding rate and outputting four data sequences; a second multiplexing means for branching each data sequence output from the second convolutional encoding means into two data sequences, and multiplexing the data sequences with a time difference inserted therebetween; and a modulation means for modulating a multiplexed data sequence generated by the second multiplexing means to generate a transmission signal. The receiver in this communication system comprises: a phase correcting means for correcting a phase of a received signal; a second diversity combining means for separating a signal output from the phase correcting means into eight data sequences, removing the inserted time difference from the data sequences, and combining the data sequences; and a Viterbi decoding means for Viterbi decoding the combined signal output from the second diversity combining means.
In a further communication system comprising a transmitter for modulating and transmitting a supplied signal, and a receiver for receiving a signal transmitted by the transmitter and demodulating the received signal according to the present invention, the transmitter comprises: a convolutional encoding means for convolutional encoding a supplied signal and outputting two data sequences; a third multiplexing means for branching each data sequence output from the convolutional encoding means into two data sequences to obtain four parallel data sequences, respectively delaying the second, third, and fourth of these four data sequences a delay time T, 2T, and 3T (where T is a specific time), and then multiplexing the data sequences; and a modulation means for modulating a multiplexed data sequence generated by the third multiplexing means to generate a transmission signal. The receiver in this communication system comprises: a phase correcting means for correcting a phase of a received signal; a third diversity combining means for separating a signal output from the phase correcting means into four data sequences, removing the inserted time difference from the data sequences, and combining the data sequences; and a Viterbi decoding means for Viterbi decoding the combined signal output from the third diversity combining means.
In a further communication system comprising a transmitter for modulating and transmitting a supplied signal, and a receiver for receiving a signal transmitted by the transmitter and demodulating the received signal according to the present invention, the transmitter comprises: a convolutional encoding means for convolutional encoding a supplied signal and outputting two data sequences; a fourth multiplexing means for branching each data sequence output from the convolutional encoding means into two data sequences to obtain four parallel data sequences, respectively delaying the second, third, and fourth of these four data sequences a delay time 2T, T, and 3T (where T is a specific time), changing the order of these data sequences, and then multiplexing the data sequences; and a modulation means for modulating a multiplexed data sequence generated by the fourth multiplexing means to generate a transmission signal. The receiver in this communication system comprises: a phase correcting means for correcting a phase of a received signal; a fourth diversity combining means for separating a signal output from the phase correcting means into four data sequences, removing the inserted time difference from the data sequences, restoring the order of the data sequences, and combining the data sequences; and a Viterbi decoding means for Viterbi decoding the combined signal output from the fourth diversity combining means.
In a further communication system comprising a transmitter for modulating and transmitting a supplied signal, and a receiver for receiving a signal transmitted by the transmitter and demodulating the received signal according to the present invention, the transmitter comprises: a convolutional encoding means for encoding a supplied signal and outputting a plurality of data sequences; a multiplexing means for branching each data sequence output from the convolutional encoding means, and multiplexing the data sequences with a time difference inserted therebetween; a modulation means for modulating a multiplexed data sequence generated by the multiplexing means; and a spectrum spreading means for spectrum spreading the modulation signal output by the modulation means to obtain a transmission signal. The receiver in this communication system comprises: a spectrum despreading means for despreading a received signal spectrum; a phase correcting means for correcting a phase of a signal output from the spectrum despreading means; a diversity combining means for separating a signal output from the phase correcting means into a plurality of data sequences, removing the inserted time difference, and combining the data sequences; and a Viterbi decoding means for Viterbi decoding the combined signal output from the diversity combining means.